This invention relates generally to multicolor image printing, and more particularly to a wide area beam sensing method and apparatus for image registration calibration in a full color printing machine.
In a typical electrophotographic printing process machine, a photoconductive member is charged to a substantially uniform potential so as to sensitize the surface thereof. A portion of the charged photoconductive member is irradiated or exposed to a light image of an document being reproduced, thereby selectively dissipating charges thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the document. The latent electrostatic latent image recorded on the photoconductive member is then developed by bringing a developer material into contact therewith. Generally, the developer material comprises charged toner particles in a liquid, or adhering triboelectrically to dry charged carrier granules or other suitable toner supporting material. During such development, the charged toner particles are attracted to the latent image forming a toner image on the photoconductive member. The toner image is then transferred from the photoconductive member to a copy sheet, and then heated to permanently affix it to the copy sheet. The foregoing generally describes a typical monochrome, for example, black and white electrophotographic printing process machine.
Several methods representing variations from the monochrome or single color process are known for producing multicolor images. In general, to produce multicolor images, different color components of a composite color image are formed and then put together in registration to achieve the composite color image. One multicolor image production method, for example, involves a process utilizing a plurality of different color toner development units, a single photoreceptor, and a multiple image frames single pass approach in which the monochrome or single color process is repeated for three or four cycles. In each cycle a component latent image of a composite multicolor final color is formed, and a toner of a different color is used to develop the component latent image. Each developed component image as such is transferred to the copy sheet. The process is repeated, for example, for cyan, magenta, yellow and black toner particles, with each color toner component image being sequentially transferred to the copy sheet in superimposed registration with the toner image previously transferred thereto. In this way, several toner component images, as are in the composite image, are transferred sequentially to the copy sheet, and can then be heated and permanently fused to the sheet.
A second method for producing color copies involves what is referred to as the tandem method which utilizes a plurality of independent imaging units for forming and developing latent component images, and a moving image receiving member such as an intermediate transfer roller or belt. In this method, the toned or developed component images from the imaging units are transferred in superimposed registration with one another to the intermediate roller or belt, thereby forming the multicolor composite image on the belt or roller. The composite image then can be transferred in one step to a sheet of copy paper for subsequent fusing.
A third method for producing color copies involves a single frame, single pass Recharge, Expose, and Develop (REaD) process. The REaD process uses a single photoreceptor, a single image frame thereon, and four imaging units each including imagewise exposure means and a development station containing a different color toner of cyan, magenta, yellow or black. A composite subtractive multicolor image can thus be produced in a single pass, and on the single frame by charging, exposing and developing, then recharging, exposing and developing again utilizing this Recharge, Expose, and Develop (REaD) process architecture. In this process, digital version of the original or document is created pixel by pixel at a computer workstation or by a scanner. When created by scanning, light reflected from the original or document is first converted into an electrical signal by a raster input scanner (RIS), subjected to image processing, then reconverted into a light, pixel by pixel, by a raster output scanner (ROS). In either case, the ROS exposes the charged photoconductive surface to record a latent image thereon corresponding to the subtractive color of one of the colors of the appropriately colored toner particles at a first development station. The photoconductive surface with the developed image thereon is recharged and re-exposed to record a latent image thereon corresponding to the subtractive primary of another color of the original. This latent image is developed with appropriately colored toner. This process (REaD) is repeated until all the different color toner layers are deposited in superimposed registration with one another on the photoconductive surface. The multi-layered toner image is transferred from the photoconductive surface to a sheet of copy paper. Thereafter, the toner image is fused to the sheet of copy paper to form a color copy of the original. The REaD process can also be performed as a multiple pass process.
In each of the color printing methods involving forming and transferring color component images in superimposed registration with one another, proper or precise registration of the images is usually an important and difficult problem. In order to deliver good quality color images, strict specifications are imposed on the accuracy with which a color image output terminal superimposes the various color separations.
Registration errors, for example, can arise from motion errors of the image receiving members, and from any mismatch between individual color separations. With respect to the motion of an image receiving member, such as that of an intermediate transfer member, good registration goals are attainable if the member is designed such that its kinematic errors are made synchronous with the spacing distance between successive points of image transfer to the member. In this manner, the modulation of its surface motion is repeatable (synchronous) with the imaging pitch and color-on-color separation errors are minimized. In such a case, even though the absolute position error of each color may be significant, the relative position error between colors can usually be held to an acceptable limit.
In tandem color image printers or output terminals, where the component color images or color separations are generated and developed on individual photoreceptors before being transferred to an intermediate belt, a mismatch in the motion errors of the photoreceptors can in addition also contribute to misregistration. A further cause of misregistration in such printers is associated with any eccentricity and wobble of the any of the photoreceptors. Motion mismatch errors for example contribute to misregistration in the process direction. Photoreceptor eccentricity contributes to variable lateral magnification errors which show up as misregistration, and wobble contributes to perceivable variations in lateral registration. Usually however, the eccentricity and wobble contributions exist only in machines where the latent image formation is performed through a finite angle by a light beam scanning system (usually called a ROS or Raster Output Scanner).
One known technique for improving registration is described in U.S. Pat. No. 4,903,067 to Murayama et al. and involves the use of a marking system and a detector for measuring alignment errors and mechanically moving individual color separation imaging units to correct misalignment. According to this technique, color printers that employ marks produced by each of the component or separation color imaging units in juxtaposition with each other, are thus enabled and able to correct lateral and longitudinal relative position, skew and magnification misregistration of the component images. The marks may be machine readable, and data may be processed to measure registration errors for the purpose of automating registration error correction. However, such corrections cannot compensate for the errors introduced by mismatch in the velocity variations of the photoreceptors because these errors differ both in phase and magnitude and are in no way steady or synchronous with the image transfer pitch. For example, a photoreceptor drum characterized by an eccentricity and wobble may rotate with an instantaneous rotational velocity that repeatedly varies as a function of the rotational phase angle such that an average rotational velocity over a complete rotation would inaccurately characterize the instantaneous rotational velocity at any single rotational phase angle.
The conventional detection system measures alignment errors in both the process direction and in a lateral direction, transverse the process direction, by detecting the position of, and determining the alignment error from the times of passage of, the centroids of registration indicia marks, such as lines, chevrons or other geometric shapes, past the centers of optical detectors, for example optical detectors. Detection and measurement of the position of each of the registration indicia marks may be accomplished by illuminating the marks and employing an optical system in an attempt to collect diffusely reflected light from the mark or transmitted light through the mark. The illumination may be in the visible wavelength or at near infrared (IR) wavelength.
The detection of color to color registration or misregistration, and the ability for correcting for detected misregistration, are very important in multicolor printing. Several techniques for doing so have been suggested and include the sensing of registration or misregistration between different color toner registration marks on a belt. One example of such techniques utilize a MOB mark or mass on belt! sensor as a first position sensor for sensing a mark or mass of toner on a moving image carrying belt. The sensor does so by detecting the position or timing of individually colored toner mass developed lines on the moving belt. A controller connected to the output of the sensor determines the differences in the timing of the sensing of each line, and from such timing information determines the relative positions of the various lines.
In U.S. Pat. No. 4,804,979 issued Feb. 14, 1989, to Kamas et al., for example, a single pass color printer/plotter including a precise registration method is disclosed in which each print station monitors registration marks to detect variations of the media during printing, and corrects for such variations. The system includes a light source, an optical sensor array comprising a pair of sensors, and an optics control unit for detecting registration marks.
Similarly, U.S. Pat. No. 4,903,067 issued Feb. 20, 1990, to Murayama et al., discloses a multi-image forming apparatus including CCD array detectors for detecting the recording positions of registration marks on a belt.
U.S. Pat. No. 4,916,547 issued Apr. 10, 1990, to Katsumata et al., discloses a color image forming apparatus including reflection sensors comprising light emitting diodes and phototransistors having circuits for producing rectangular output wave signals.
U.S. Pat. No. 4,963,899 issued Oct. 16, 1990, to Resch, III, discloses a method and apparatus for image frame registration utilizing line indicia marks on an intermediate transfer belt, and bi-cell sensor arrays including photoemitter/photosensor pairs for detecting the indicia marks.
U.S. Pat. No. 4,965,597 issued Oct. 23, 1990, to Ohigashi et al., discloses a color image recording apparatus that superimposes a plurality of images having different colors to form a composite color image on a recording medium. Registration marks are formed on the recording medium at equal pitches. This occurs when it is transported through an image formation device in the apparatus. The apparatus also includes a sensor for sensing the registration marks and an edge sensor for sensing one edge or both edges of the recording medium. The mark sensor includes a source of light and a light receiving photosensor comprising a phototransistor, amplifiers and control circuits.
U.S. Pat. No. 5,278,587 issued Jan. 11, 1994, to Strauch et al., discloses a method and apparatus for color image on image registration utilizing a detector placed beneath the photoreceptor belt to provide a signal representing the exposure level of each scanning beam. Timing information derived from the detectors is used to control registration of the first scan line of each image sequence.
U.S. Pat. No. 5,287,162 issued Feb. 15, 1994, to deJong et al., discloses a method and apparatus for sensing and correcting image on image registration errors. The method and apparatus include use of bi-cell detectors or CCD array detectors for determining the timing of the passing of toner marks under the sensors.
Pending U.S. application Ser. No. 07/354,305 (Attorney docket D/92054) entitled "METHOD TO PROVIDE OPTIMUM OPTICAL CONTRAST FOR REGISTRATION MARK DETECTION" and Ser. No. 08/168,300 (Attorney docket D/94529) entitled "METHOD AND APPARATUS TO IMPROVE REGISTRATION IN A BLACK FIRST PRINTING MACHINE" each disclose a color image on image registration method and apparatus utilizing a bicell detector comprising a photoemitter photosensor pair for detecting toner registration marks on a photoreceptor belt.
U.S. Pat. No. 5,394,223 issued Feb. 28, 1995 to Hart et al, and commonly assigned, discloses a printing device for providing color prints of the type having a semi-transparent imageable surface adapted to move along a preselected path. The printing device also has at least one image processing station for forming a composite image on the imageable surface; means for marking indicia on the imageable surface; means for sensing the indicia to detect registration deviations from the preselected path of movement of the imageable surface; and means, responsive to the sensing means, for adjusting the image processing station to compensate for the detected registration deviations, thereby enhancing the registration of the composite image on the imageable surface. The sensor disclosed is a fixed position sensor that is located on the back side of a translucent moving image carrying belt, and directly opposite the point of ROS exposure of the charged front side of the moving belt. As such, at a non-black ROS/Imaging station (in a black first REaD printer), a previously developed black image on the front side of the belt will occlude or block the ROS exposure light from the backside sensor, thus providing timing information for proper registration of the black image and the non-black image of the particular imaging station.
Unfortunately, the MOB sensor and Eclipse sensor techniques as disclosed in the above references are based on a timing parameter, and therefore each requires exact and precise timing measurements. For such measurements, each therefore requires that the marks, lines or image edges being sensed be formed precisely, and be of high quality development. Such all around required precision necessarily demands high precision and costly sensors as well as costly electronics or controllers.